Commercial processes for the manufacture of vinyl aromatic compounds such as monomeric styrene, divinyl benzene and lower alkylated styrenes (such as alphamethylstyrene and vinyltoluene) typically produce products contaminated with various impurities, such as benzene, toluene and the like. These impurities must be removed in order for the monomer product to be suitable for most applications. Such purification of vinyl aromatic compounds is generally accomplished by distillation.
However, it is well known that vinyl aromatic compounds polymerize readily and that the rate of polymerization increases rapidly as the temperature increases. In order to prevent polymerization of the vinyl aromatic monomer under distillation conditions various polymerization inhibitors have been employed.
In general, the compounds which are commercially employed as such polymerization inhibitors are of the U.S. Pat. No. 2,526,567, show the stabilization of nuclear chlorostyrenes employing 2,6-dinitrophenols. Similarly, U.S. Pat. No. 4,105,506, to Watson, discloses the use of 2,6-dinitro-p-cresol as a polymerization inhibitor for vinyl aromatic compounds.
In addition, it has been disclosed by Butler et al, in U.S. Pat. No. 4,466,905, that, in the presence of oxygen, the presence of phenylenediamines in the distillation column with 2,6-dinitro-p-cresol will further reduce the amount of polymerization which occurs.
While dinitrophenols are effective polymerization inhibitors, there are several disadvantages associated with their use, either alone or in blends. For example, dinitrophenols are solids that, if subjected to temperatures above their melting points, are unstable and may explode (see U.S. Pat. No. 4,457,806).
Moreover, dinitrophenols are highly toxic, many having an LD.sub.50 (rat) of less than 30 mg/Kg (Sax, Hazardous Properties of Industrial Chemicals).
Recently, it has been disclosed by Kolich, in U.S. Pat. No. 4,633,026, that halogenated vinyl aromatic compounds (such as bromostyrene) may be inhibited from polymerizing by the addition of an amine polymerization inhibitor selected from the group consisting of certain alkyl-substituted phenylenediamine compounds and phenothiazine compounds in the presence of air.
While such prior art inhibitors may inhibit the polymerization of vinyl aromatic compounds to some degree, it would be desirable to possess polymerization inhibitors which would more effectively delay the onset of polymerization and/or which would avoid the use of highly toxic compounds such as dinitrophenols.
Accordingly, it is an object of this invention to provide an improved inhibitor composition for the prevention of polymerization of vinyl aromatic compounds.
It is an additional object of this invention to provide an inhibitor for the prevention of polymerization of vinyl aromatic compounds, which inhibitor does not comprise toxic dinitrophenolic compounds.
It is a further object of this invention to provide a vinyl aromatic composition which is stabilized against polymerization.
It is yet another object of this invention to provide an improved method for inhibiting the polymerization of vinyl aromatic compounds.
The foregoing and additional objects will become more fully apparent from the following description and accompanying Examples.